1. Field of the Invention
The present invention relates to a process for producing a self-restoring overcurrent protective device which undergoes heat buildup upon flowing therethrough of an overcurrent to increase the resistance thereof to thereby limit the current and is reversibly self-restored stably to an original state thereof upon returning of a circuit to a normal state thereof by utilizing the phenomenon of the same as a positive temperature coefficient thermistor (hereinafter referred to briefly as the "PTC").
2. Description of the Prior Art
Conventional processes for producing a resistor or a device which utilizes the PTC characteristics thereof include the following ones:
(i) a process for producing a resistor having PTC characteristics which comprises sintering a semiconductor of a barium titanate type at a high temperature to form a device;
(ii) a process for producing a relatively low resistance PTC device comprising a polymer substance and carbon black incorporated thereinto, an example of which is a process for producing a device having PTC characteristics as disclosed in U.S. Pat. No. 4,237,441 which comprises molding a mixture of a crystalline polymer and carbon black into a predetermined shape with an extruder and irradiating the resulting molding with a radiation such as electron beams to crosslink the crystalline polymer between the molecules thereof to form a network structure, thereby improving the molding in the thermal deformation thereof;
(iii) a process for producing a resistor having PTC characteristics as disclosed in, for example, Japanese Patent Laid-Open No. 8,443/1981 which comprises molding a mixture of a rubbery substance, carbon black, graphite, an organic peroxide, and the like into a predetermined shape and heating the resulting molding to decompose for the first time the organic peroxide whereupon a network structure is given to the rubbery substance to improve the molding in the thermal deformation thereof; and
(iv) a process for producing a resistor having PTC characteristics as disclosed in, for example, Japanese Patent Publication No. 36,876/1976 which comprises graft-copolymerizing a vinyl monomer onto carbon black in a solvent, adding a cross-linking agent to the resulting kneaded mass, and heating the resulting mixture to give a network structure thereto for attaining an improvement in the thermal resistance thereof.
The above-mentioned conventional process (i) comprising sintering a semiconductor of a barium titanate type at a high temperature involves problems that, since the resulting device has a high volume resistivity, the voltage drop of a circuit at a steady-state current is large, that, when the temperature of the device is further raised after the manifestation of PTC characteristics, the device turns into a negative temperature coefficient thermistor (hereinafter referred to briefly as the "NTC") so that the current-limiting function thereof is drastically reduced, and that scattering of resistance values is liable to occur due to the deformation of the device caused by sinter molding at a high temperature.
The above-mentioned conventional processes (ii) and (iii) comprising crosslinking a crystalline polymer substance admixed with carbon black or a rubbery substance admixed with carbon black and graphite provide a thermally stable PTC material as a heater which acts as an overcurrent protective device, but involve a problem that part of carbon black particles or part of carbon black and graphite particles move due to segment expansion and contraction in a crosslinked network structure during the course of repeated current-limiting actions of the PTC device as the overcurrent protective device to lower the reproducibility of PTC characteristics and resistance value between the repeated actions and particularly to largely vary the resistance value therebetween.
The above-mentioned conventional process (iv) comprising graft-copolymerizing a vinyl monomer onto carbon black in a solvent involves a problem that the compatibility of the resulting crystalline polymer substance with the solvent during the course of graft copolymerization is so problematic because of the use of the solvent in the graft copolymerization that polyethylene and polypropylene which are crystalline polymer substances effective in manifestation of PTC characteristics cannot be employed.
It is known to use an organic peroxide, such as dicumyl peroxide, as a network-forming agent for an ethylene-propylene rubber and the like. Where such an organic peroxide is added to a rubber and they are roll-milled, roll milling is conducted at a comparatively low temperature, for example, around 50.degree. C., for the purpose of preventing gelation (network formation) during the course of milling. In an unavoidable case, particularly in the case of using a crystalline substance such as polyethylene, a method like one in which addition of the organic peroxide is completed in a comparatively short time is employed with consideration given to an indication of the thermal decomposition rate of the organic peroxide, namely the half-life thereof. This is done for the purpose of suppressing the decomposition of the organic peroxide as much as possible during the course of milling.
Accordingly, milling of a polymer substance with an organic peroxide at or above the thermal decomposition temperature thereof to allow both to react with each other during the course of milling has heretofore been avoided as much as possible.
Meanwhile the inventors of the present invention have found an interesting fact that, when an adequate amount of an organic peroxide is added while milling a crystalline polymer substance in the presence of graphite and carbon black, the organic peroxide does not serve as a crosslinking agent for the polymer but, instead, acts as a grafting agent to enable the polymer to be grafted onto the surfaces of graphite and carbon black particles even during the course of milling at or above the thermal decomposition temperature of the organic peroxide.